Support for Sen. Elizabeth Warren and her liberal populist views has become a litmus test for Democratic candidates seeking to claim the progressive mantle in their primaries in 2014.

Jon Cowan and Jim Kessler of Third Way, a centrist Democratic think tank, penned an op-ed in The Wall Street Journal on Monday bashing Warren, D-Mass., and New York City Mayor-elect Bill de Blasio for what they call their "disastrous" embrace of economic populism. In the op-ed, the pair specifically refers to the existence of "the Warren wing of the Democratic Party."

That hasn't sat well with progressive activists. And Wednesday, liberal groups including MoveOn.org, the Progressive Change Campaign Committee, Democracy for America, and Progressives United started directing their ire at not just Third Way but Rep. Allyson Schwartz, D-Pa., an honorary cochair of the group who is also the nominal front-runner in the Pennsylvania gubernatorial primary. In effect, they told the congresswoman she had to choose Warren over Third Way if she wanted their support.

"If Allyson Schwartz opposes cutting the benefits that seniors and working families rely on and earn with every single paycheck, now is the time for her to say so publicly and resign from her position at Third Way," said Jim Dean, DFA's chairman.

And on Tuesday, Schwartz's rivals started piling on. John Hanger, one of Schwartz's seven primary opponents, is also joining the call for Schwartz to resign her post with the group.

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The liberal Left's action has already earned results: Schwartz, while not resigning from Third Way, did call its op-ed "outrageous."

Progressives see an attack on Warren, no matter how many degrees removed, as unacceptable for any candidate who wants the backing of the activist Left in his or her primary. And it's a phenomenon not confined to Pennsylvania. That carries big implications for all Democratic candidates, especially those in primary fights, in 2014.

In an attention-grabbing cover story in November, Noam Scheiber at New Republic argued that come 2016, Democrats unsatisfied with a Hillary Rodham Clinton candidacy may turn their sights to Warren as the party's populist, progressive standardbearer. But if the Third Way fiasco is any indication, many already have.

Already in 2013, Democrats in competitive races in Maryland, Rhode Island, New Jersey, Massachusetts—and now Pennsylvania—have invoked Warren's name or ideas in an attempt to emulate the freshman senator who has proven to be so popular with their base.

When asked by MSNBC's Perry Bacon back in September what he thought about President Obama's nomination of Janet Yellen to lead the Federal Reserve, now-Sen. Cory Booker, D-N.J., noted that "senators I like a lot, like Elizabeth Warren," were angling for Yellen. Bacon attributed this name-check to Booker simply doing "what is necessary to illustrate his liberal bonafides" in case he himself ever decides to run for president.

In Maryland, Attorney General and 2014 gubernatorial candidate Doug Gansler pitched a replica of the "People's Pledge" in his own Democratic primary contest, which was used by Warren and then-Sen. Scott Brown, R-Mass., to limit outside spending in their 2012 U.S. Senate race. In an email sent to supporters Nov. 21, Gansler's spokesman wrote: "Our position is clear and we're happy to stand with Elizabeth Warren on this issue."

Providence Mayor Angel Taveras also proposed such an agreement in the Democratic gubernatorial primary in Rhode Island, as did state Treasurer Steve Grossman in the Democratic gubernatorial primary in neighboring Massachusetts. When Taveras unveiled his proposal in September, he said: "I think Elizabeth Warren set a terrific example for Democrats by calling for an end to outside spending in campaigns."

Grossman, a former DNC chairman, has nearly as much cash in his campaign account as all of his Democratic primary opponents combined and appears to have pursued the pledge in a genuine attempt to emulate Warren and stake out progressive ground in the deep blue state.

The emergence of Warren and what she represents as a progressive litmus test bears a striking resemblance to the way conservative groups have held the views of Republican candidates up against the hard-right stances of Sen. Ted Cruz of Texas. And presidential bid or no, Warren's bold liberalism is proving itself to be a calling card for a branch of the party—a branch potentially decisive in 2014 primaries—tired of pragmatic progressivism and yearning for a new way forward.